1. Field of the Invention
This invention relates to a system for adding a controlled amount of air to exhaust gases in the exhaust manifold of an internal combustion engine downstream of the combustion chamber for the purpose of cleaning the exhaust gases.
2. Description of the Prior Art
In this type of exhaust cleaning system, the flow rate of additional air to be fed to the engine exhaust manifold from an air pump is controlled by a flow control valve in response to vacuum signals from an on-off solenoid valve which is under on-off duty control of a microcomputer operating on the input signals indicative of variations in the partial oxygen pressure in the exhaust gases as detected by an oxygen sensing element, the engine r.p.m., the exhaust gas temperature, the throttle opening of the carburetor and the intake manifold vacuum. The cleaning system normally employs a vacuum control valve for providing a vacuum source of a predetermined level by controlling the vacuum which is drawn from the engine intake manifold. The controlled vacuum is fed to the on-off-duty solenoid valve thereby producing vacuum signals to operate the flow control valve. Namely, the feed rate of the addition air to the engine exhaust gases is solely controlled by the on-off-duty solenoid valve operating in response to command signals from a microcomputer which processes various operational variables according to predetermined arithmetic formulas. In this connection, recently there is an increasing trend toward a greater reliance on the control and arithmetic operations by the microcomputer. Therefore, it is desirable to lessen the burden which is imposed on the microcomputer for the control and arithmetic operations, even in a sophisticated control system.
Further, in the conventional control method employing a vacuum control valve which produced output vacuum of a constant level, the vacuum control in response to the engine load has been effected solely through the control of the air bleed amount by the on-off solenoid valve, thus involving problems concerning large air consumption and vacuum losses.